1. Field of the Invention
The present invention relates to a liquid crystal display device with a backlighting unit, more specifically to an LED backlighting unit for a liquid crystal display panel.
2. Description of the Related Art
A liquid crystal display device used in a personal computer or a cellular telephone has a backlighting unit for a liquid crystal display (LCD) panel. A cold cathode fluorescent lamp has been used as a white light source for the backlighting unit. Recently, however, an array of red, green and blue light emitting diodes (LEDs) with a color-mixing optical waveguide is proposed as the white light source for an LED backlighting unit in view of its long lifetime and excellent light emission.
When the LED backlighting unit is used for a medium-sized or large-sized LCD panels, it is necessary to apply a large number of LEDs in order to gain required luminance. The numbers of the LEDs for the red, green, and blue colors may be different from one another in order to obtain required luminance values. When using the LEDs of these three colors, it is necessary to mix the respective colors appropriately to emit a uniform white color toward a display surface. Accordingly, it is necessary to use a color-mixing optical waveguide in addition to a conventional main optical waveguide.
Examples of a liquid crystal display device with the LED backlighting unit are disclosed in Japanese Unexamined Patent Publications Nos. 2004-199967 and 2004-118205.
FIG. 1 is a cross-sectional view showing a configuration of a liquid crystal display device of this type. As shown in FIG. 1, a color-mixing optical waveguide 4 is disposed under an optical wave guide 5 substantially in parallel. An LED 1 and a reflector 3 are disposed under a substantially central part of the optical waveguide 5 and in the vicinity of an incident surface of the color-mixing optical waveguide 4. As to the LED 1, three types of LEDs respectively configured to emit the wavelengths corresponding to the red, green, and blue colors are used as the light source.
The reflector 3 reflects light emitted from the LED 1 and guides the light to the color-mixing optical waveguide 4. A semi-cylindrical reflector 6 is disposed at an emitting surface side of the color-mixing optical waveguide 4. Surfaces of the reflectors 3 and 6 are formed into mirror planes so as to reflect the light efficiently.
To reflect the light toward the optical waveguide 5, the reflector 6 is provided so as to surround the emitting surface of the color-mixing optical waveguide 4 and a side surface of the optical waveguide 5. An optical sheet 7 and an LCD panel 8 are disposed on a display surface side of the optical waveguide 5. A reflection sheet 9A is disposed on the opposite side to the display surface side of the optical waveguide 5. The reflection sheet 9A is bent at an end portion of an aluminum frame 9B, and is fitted to an upper surface of the color-mixing optical waveguide 4 by use of a fixture 9C.
The optical sheet 7 includes a diffusion sheet, a protection sheet, a prism sheet, and the like. The optical sheet 7 diffuses or condenses the incident light and emits the light toward the LCD panel 8.
The LCD panel 8 applied hereto may be similar to a conventional LCD panel. Therefore, description of the LCD panel 8 will be omitted. Light propagation in the liquid crystal display device will be described with reference to FIG. 1. The light from the LED 1 is made incident onto the incident surface of the color-mixing optical waveguide 4 either directly or after being reflected by the reflector 3. The incident light on the incident surface of the color-mixing optical waveguide 4 is propagated and subjected to color mixing inside the color-mixing optical waveguide 4. The light sufficiently mixed together and converted into the white light is emitted from the emitting surface of the color-mixing optical waveguide 4 and is then incident on the reflector 6. The reflector 6 reflects the light emitted from the color-mixing optical waveguide 4 toward the optical waveguide 5.
The incident light on the optical waveguide 5 spreads uniformly over the entire surface thereof, and is emitted toward an emitting surface provided with the optical sheet 7 and the LCD panel 8. A sheet or a tape made of polyethylene terephthalate (PET), polycarbonate, or the like on which silver or the like is evaporated, a white sheet, a white tape, and the like is used as the reflection sheet 9A. The reflection sheet 9A reflects the light emitted from a lower surface of the optical waveguide 5 causing the light to be incident again on the optical waveguide 5. In this way, the reflection sheet 9A improves light use efficiency.
However, the liquid crystal display device shown in FIG. 1 has the following problems. First, the LED 1 and the reflector 3 cannot maintain close contact due to variation in terms of dimensions and a position of fixation of the reflector 3. As a consequence, a gap is caused between the LED 1 and the reflector 3, and the light leaks out from the gap and luminance efficiency is thereby deteriorated. Second, the reflector 3 made of metal is located close to the LED 1. Accordingly, a large stress may be applied from the metal reflector 3 to the LED 1 due to vibrations or impacts, and the LED 1 tends to be damaged easily.